Electroluminescent organic materials can be classified into two categories: conjugated polymers and organic small molecules.
Polymeric electroluminescent organic materials include poly(1,4-phenylenevinylene)s, polyfluorenes, and their derivatives. Electroluminescent polymers are attractive because of their solution processability, which is a relatively cost effective method for manufacturing electronic devices containing electroluminescent organic materials.
However, high purity is difficult to achieve for polymeric light emitting materials since such materials often contain certain amounts of structural defects in the polymer backbone, by-products produced during polymerization, and end groups remaining on the polymer chains. All these impurities and/or defects in preparations of the polymers can affect efficiency and lifespan of electronic devices incorporating polymeric electroluminescent organic materials.
Small molecules represent another category of light emitting materials and have been widely used in organic light emitting devices, such as organic light emitting diodes (OLEDs), either as emitters or as charge transporting materials. It is possible to produce fairly pure small molecule light emitting materials due to use of purification processes such as sublimation or recrystallization process, which provides an advantage compared with light emitting polymers.
However, devices containing small molecules are typically processed using vacuum deposition techniques, which tend not to be cost-effective and which may not be desirable for mass production. Generally, solution processing is a lower cost technique and is more suitable for mass and fast production, and may also be better suited for preparation of larger films that are required for large displays.
Multilayer devices are known, and are typically constructed with a hole transport layer, an emissive layer and an electron transport layer, with possible inclusion of a hole injecting layer and/or electron injecting layer.
However, many of the current organic light emitting materials, both polymers and small molecules, typically have imbalanced charge transporting characteristics. Generally, light emitting materials are able to conduct only one charge carrier, either holes or electrons, but typically not both. For example, poly(1,4-phenylenevinylene)s or alkoxy-substituted poly(1,4-phenylenevinylene)s are good hole transporters, whereas tris-(8-hydroxyquinoline) aluminum (III) (Alq3) is an electron transporter. Imbalanced charge transporting in OLED devices results in low device efficiency.
One solution to balance the charge carriers in the devices, is to combine both electron transport segment and hole transport segment into one structure to construct an ambipolar material; either the two segments are linked together in the backbone or they are attached to the backbone separately.
Although some such ambipolar materials have been developed, to date the reported device performance based on such materials is still not satisfactory.
Thus, there exists a need for new materials that can be used in an organic light emitting layer in an electroluminescent device.